The mechanisms that control the growth and functional differentiation of the liver in the late gestation fetus are critical to fetal metabolis and metabolic adaptation of the newborn. Their perturbation contributes to fetal origins of adult metabolic disorders. The biology of fetal liver development also has relevance for cell-based therapy for liver diseases, hepatic carcinogenesis, and the control of fetal somatic growth. This proposal is based on the central hypothesis that late gestation fetal liver development is regulated by mechanisms that differ from those that control adult liver mass. It focuses on the mechanisms by which the mammalian Target of Rapamycin (mTOR), a nutrient-sensing Ser/Thr kinase, regulates cell cycle progression, ribosomal biogenesis, protein synthesis, and gene expression. The long term goal of the project is to understand nutrient regulation of fetal somatic growth. This goal will be pursued through three Specific Aims. The first two will focus on molecular mechanisms that control cell proliferation and growth. Results from these aims will be applied to the third aim that will employ a novel animal model of enhanced mTOR signaling. Specific Aim 1 is to determine the mechanism by which mTOR controls cell cycle progression through regulation of the activity of cyclin E-containing complexes. This aim will test the hypothesis that mTOR mediates post-translational modification of components of cyclin E complexes that regulate activity of the cyclin E effector, cyclin- dependent kinase 2 (CDK2). Specific Aim 2 is to test the hypothesis that the Ser/Thr protein phosphatases, PP2A and PP6, are directly involved in the mTOR-mediated regulation of hepatocyte proliferation, ribosomal biogenesis, and translation. In Specific Aim 3, we will apply the findings from the first two aims to a model of global mTOR activation in the late gestation fetal mouse. We will use a genetic mouse model in which there is conditional deletion of a key mTOR inhibitor, Tsc1. This aim will test two hypotheses. The first is that fetal cells in a spectrum of tissues are responsive to augmented mTOR activity through augmented cell cycle progression, ribosomal biogenesis and global protein synthesis, and through modulation of gene expression. The second is that mTOR activation can rescue the fetus from the growth retardation associated with maternal protein restriction. The project will incorporate innovative analytical approaches, including stable shRNA- mediated gene deletion and mass spectrometry-based phosphoproteomics, to characterize molecular mechanisms involved in mTOR signaling. Completion of these aims will advance our understanding of how nutrients regulate fetal growth during late gestation. Our results will provide insight into the pathogenesis and prevention of dysregulated liver metabolism and liver cancer while also contributing to the development of cell- based strategies for therapy of liver disease. The proposed work also has significance for several broad areas of human health and disease, including the dysregulation of fetal growth and its attendant postnatal consequences, the biology of facultative progenitor cells, and tissue regeneration and carcinogenesis.